The present invention relates to an optical scanner for use with a laser beam printer and other optical image forming apparatus. More specifically, the invention relates to a method of compensating for the deterioration in optical characteristics due to temperature variations.
Prior Art:
A prior art optical scanner is shown schematically in FIG. 10. A laser beam, or rays of light issuing from a semiconductor laser 211, is collimated to parallel light by means of a collimator lens 221 and shaped to a convergent beam by means of a cylindrical lens 222. The convergent beam is convergent in one of the cross sections which are orthogonal to each other and include the optical axes of collimator lens 221. The shaped beam is deflected by a rotating polygonal mirror 230. The deflected beam is passed through scanning lenses 251 to be focused as a spot on the surface to be scanned 240. The image point is substantially equal to the beam waist of a Gaussian beam. To insure that the beam is focused to form a planar image on the surface 240, the scanning lenses 251 is designed such that it will not produce astigmatism or curvature of the field in greater amounts than specified values. In addition, the scanning lenses 251 is designed to create negative distortion so that the beam deflected from the rotating polygonal mirror 230 at uniform angular velocity will scan the surface 240 at uniform linear velocity.
If a ray of light incident on the scanning lens 251 at a viewing angle of .theta. is transformed to form an image in such a way that the height of the image y is proportional to .theta., the relationship expressed by y=f.multidot..theta. will hold, where f is the focal length of the lens 251. A lens satisfying this relationship is commonly referred to as an "f.theta. lens".
The convergent beam from the cylindrical lens 222 forms a line image on a deflecting surface of the rotating polygonal mirror 230 in a direction parallel to the scanning direction. The line image will eventually provide a spot of a specified size on the surface 240. Hence, a sub-scanning cross section of the optics under consideration is such that each of the deflecting surfaces of the rotating polygonal mirror 230 is optically conjugated with the surface 240. This means that even if the deflecting surfaces of the polygonal mirror 230 are not uniformly parallel to the axis of rotation such that the angle of the deflected beam fluctuates with a specific deflecting surface in the sub-scanning direction, the conjugated relationship is maintained and successive beam spots are formed at the same position on the surface 240 in the sub-scanning direction. Optics of this type is commonly called "tilt-correcting optics" on account of its ability to correct the tilting of deflecting faces of the rotating polygonal mirror 230. Wherein the term "main scanning direction" means the direction which is swept by the light beam which is deflected by the rotating polygonal mirror and the term "sub-scanning direction" means the direction which is perpendicular to the main scanning direction and the optical axis of lenses.
In order to insure that the line image on a deflecting surface produces a circular or elliptical spot of a specified size on the surface 240, the scanning lenses 251 must have different optical characteristics in the main and sub-scanning directions. Optics of this type are commonly called "anamorphic optics".
In most of the conventional optical scanners, the scanning lenses 251 has totally been composed of glass lens elements in order to assure exact precision. However, the use of injection-molded plastic lens elements is increasing today because of the great latitude in shape that is offered by injection molding and for economic reasons. If toric surfaces are to be used in constructing "anamorphic" optics and, particularly in the case of providing them from aspheric (or non-arcuate) rather than arcuate cross-sectional shapes, fabrication of the desired lens elements at a practically feasible cost by working on glass is very difficult and can only be commercialized by the use of plastic materials.
One of the major applications of optical scanners is on laser beam printers and efforts are constantly being made to provide better resolution. To this end, the size of the beam spot to be formed on the surface 240 must accordingly be reduced. Therefore, the first requirement to be met is to design optics having high imaging performance; in addition, it is necessary to assemble an optical scanner with the positions and other features of respective lenses being precisely adjusted such that a light beam is properly focused on the surface to be scanned in both a main and a sub-scanning cross section.
Another problem with such high-resolution optics is that even if they have initially the intended imaging performance, the position in which the light beam is focused to form an image will shift axially on account of environmental variations, such as temperature changes, and this "defocusing" phenomenon may occasionally deteriorate the imaging performance of the optics. This problem has heretofore been addressed by the proposal of various correcting or compensating mechanisms.
Problems to be Solved by the Invention:
However, the proposals made so far have not been completely satisfactory in preventing the deterioration of imaging performance due to environmental variations, particularly temperature changes. The problems involved in the respective prior art techniques will now be described with reference to the patent literature.
Unexamined Published Japanese Patent Application (kokai) Sho 55-43577 teaches a technique in which optical parts around the collimator lens are designed to have appropriate linear expansion coefficients such that the temperature-dependent variations in the distance from the collimator lens to the semiconductor laser are reduced to within the depth of focus of the collimator lens, thereby ensuring against temperature-dependent variations in the characteristics of the beam issuing from the collimator lens. The thermal expansion of the optical parts around the collimator lens is considered by this technique but it does not take into account other factors such as the changes in refractive index due to wavelength variations in the light source and the dispersion of the lenses used, the refractive index variations with the constituent material of the lenses and the thermal expansion of the lenses. In actual optics, temperature can also affect these factors, thereby causing an image to be formed at a point distant from the surface to be scanned.
If the numerical aperture of the collimator lens is small, the depth of focus is large enough (e.g., several tens of micrometers) to present no significant problems even if the thermal expansion coefficients of optical parts around the collimator lens are not selected with particular care. In practice, however, any variation in the distance from the light source to the collimator lens is amplified by the longitudinal magnification of the overall optics placed between the light source and the surface to be scanned and the position of image formation will depart greatly from the intended image plane. In addition, the primary objective of the technique is to insure that the light beam emerging from interchangeable laser units will have constant characteristics and it does not intend to compensate for the temperature-dependent changes in the imaging performance of the scanning optics by causing deliberate temperature-dependent changes in the properties of the light beam issuing from the collimator lens, as will be described later in this specification.
Unexamined Published Japanese Patent Application (kokai) Sho 63-7530 teaches a technique that addresses the problem of temperature-dependent variations in the operating wavelength of a semiconductor laser which was not considered by the invention described in Unexamined Published Japanese Patent Application (kokai) Sho 55-43577, supra. The technique consists of providing the collimator lens with specified chromatic aberrations by utilizing the dispersion of glass such that any variation in the focal length of the collimator lens that will experience index variations due to the introduced chromatic aberrations can be canceled by the thermal expansion of the optical parts that couple the collimator lens of the semiconductor laser. However, this proposal also fails to take into account the temperature-dependent variations in the refractive index of the lens and its thermal expansion.
This technique parallels the invention described in Unexamined Published Japanese Patent Application (kokai) Sho 55-43557 in that the primary objective is to ensure that the light beam issuing from the collimator lens will maintain a constant (say, parallel) state irrespective of temperature. However, in order to compensate for the temperature-dependent variations in the characteristics of lenses other than the collimator lens, the state (in particular, the angle of divergence) of the beam issuing from the collimator lens is desirably varied with temperature. This is particularly true in the case where the scanning lens is made of a plastic material. Since plastic materials will experience temperature-dependent variations in refractive index that are about ten times as great as the index variation in glass, it is very difficult to compensate for such great variations by lenses other than the collimator lens if the beam issuing from the latter is held in a constant state irrespective of temperature.
A method for correcting the temperature-dependent variation in the refractive index of a plastic lens by utilizing certain parameters is described in Unexamined Published Japanese Patent Application (kokai) Hei 3-163411. According to the method, the scanning lens is made of a plastic material and the temperature-dependent variation of its refractive index and the variation of its focal length due to its own thermal expansion are subjected to optimal correction by taking into consideration the change in the distance from the semiconductor laser to the collimator lens, as well as the wavelength variation in the semiconductor laser.
However, this method also does not take into account the temperature-dependent variation in the refractive index of the collimator lens or its thermal expansion and, hence, is not an ideal means for achieving satisfactory correction for potential temperature changes. In addition, none of the three prior art techniques described above provide an effective solution to the problems involved in the correction of optics, such as the aforementioned scanning optics equipped with "tilt correcting" optics, that experience different amounts of temperature-dependent changes in imaging characteristics in the main and sub-scanning directions. There have been no prior art methods that take into account two types of deviation of the image plane from the surface to be scanned, one being due to curvature of the field and astigmatism and the other being the deviation on account of temperature variations.